1. Field of the Invention
The present invention relates to a method for manufacturing a glass blank for magnetic disk, a method for manufacturing a glass substrate for magnetic disk, a glass blank for magnetic disk, a glass substrate for magnetic disk and a magnetic disk.
2. Description of the Related Art
Recently, a hard disk device (HDD) is incorporated in a personal computer or a DVD (Digital Versatile Disc) recording apparatus in order to record data. Particularly, in the hard disk device used in an apparatus such as the notebook personal computer based on portability, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface. A glass substrate is suitably used as the substrate for magnetic disk because the glass substrate hardly plastically deformed as compared to a metallic substrate (aluminum substrate) and the like.
The magnetic head includes, for example, a magnetic resistance effect element, but such a magnetic head may cause a thermal asperity trouble as its specific trouble. The thermal asperity trouble is a trouble in which when a magnetic head passes over a micro-irregularly-shaped surface of a magnetic disk while floating and flying, a magnetic resistance effect element is heated by adiabatic compression or contact of air, causing a read error. Thus, for avoiding the thermal asperity trouble, the glass substrate for magnetic disk is prepared such that surface properties, such as the surface roughness and flatness, of the principal face of the glass substrate are at a satisfactory level.
As a conventional method for manufacturing a sheet glass (glass blank), a vertical direct press method is known. This press method is a method in which a lump of molten glass is fed onto a lower die, and the lump of molten glass (molten glass lump) is press-formed using an upper die (Japanese Patent Laid-open Publication No. 1999-255521) Furthermore, there is known a horizontal direct press method for sandwiching a falling molten glass in the horizontal direction to press-form the same (Japanese Patent Publication No. 4380379, FIG. 4 etc).
As a conventional method for manufacturing a glass substrate for magnetic disk, a method is known in which a glass blank formed by press-forming a molten glass lump is subjected to annealing treatment. The annealing treatment is a treatment for releasing a strain (internal strain) generated within a glass blank by holding the press-formed glass blank for a predetermined time period at a temperature which is no lower than a strain point and no higher than a glass transition point (Tg) (Japanese Patent Laid-open Publication No. 2008-287779). The strain point is a temperature at which the internal strain of glass disappears in several hours, and at which the viscosity of glass is about 1014.5 dPa·s.